A servomechanism, sometimes shortened to servo, is an automatic device that uses error-sensing negative feedback to correct the performance of a mechanism and is defined by its function. It usually includes a built-in encoder. A Servomechanism is sometimes called a ‘Heterostat’ since it controls a system’s behavior employing Heterostasis.
The term “Servo system” correctly applies only to systems where the feedback or error-correction signals help control mechanical position, speed, or other parameters. For example, an automotive power window control is not a servomechanism, as there is no automatic feedback that controls position—the operator does this by observation.
By contrast, a car’s cruise control uses closed-loop feedback, classifying it as a servomechanism.